About Crystal Ltd.
Crystal Ltd. was established in 1995 to produce exceptional quality bismuth telluride alloys that are used to make thermoelectric modules. Since 2004, Crystal Ltd. has been directly manufacturing TE modules, with their (process patented) materials and now has a wide range of thermoelectric modules for cooling (warming and thermal cycling) and for power generation. The performance of the modules, like their TE material, are among the very best in the world. The company also builds a variety of thermoelectric assemblies.
Crystal Ltd. uses a, directed crystallization from melt, method known as a, "modified Bridgeman method," for the production of both the "P" and "N" type Bi2Te3 materials.
Essentially, plates of the "P" & "N" material are produced in the desired thickness that are then sliced and diced into the desired footprint for use in TE modules. The actual crystallization of the material, in plate form, takes place in graphite molding dies that follow an ideal, and precise, temperature reduction, from melt. The process allows for crystallization to take place from the front, along melted material during cooling. After crystallization, the plates have a preset geometry with highly uniform material. The cleavage planes within the plates are substantially parallel to plate's surface which ensures highly efficient material with better mechanical properties than the TE material produced by the conventional ingot process.
The Young's modulus value is from 3700 up to 6700 MPa, much less than Young's modulus value of materials produced by the traditional methods, which is from 7700 up to 18000 MPa. Materials produced with the "directed crystallization from melt" method also have a higher maximum strain before destruction value of about 0.6% in contrast to 0.25% for materials produced by the traditional method.
Seeking uniformity in TE module performance?
The TE material produced in the conventional process has different characteristics at various points along the length of the ingot, with the greatest differences seen between the center and the ends. Historically, particularly demanding buyers have sought modules made with dice from a "center slice" of the ingot. In order to provide modules with similar performance some companies, producing TE materials conventionally, sort the dice yield from each ingot into groups with similar characteristics. Naturally the tighter the tolerance, the fewer dice that fit within the acceptable range and are not necessarily identical.
The Crystal Ltd. process allows growing a batch/lot of several dozen plates of TE material simultaneously. With complete control of the material properties and homogeneous results, the material can be used in modules with exceptionally similar characteristics. The process produces very similar material from lot to lot but, the material from individual lots is essentially identical, and traceable. This is particularly valuable to OEMs that use multiple modules in the same instrument and require consistent performance from each module. Lapping to a close height tolerance of 0.01 mm is available.
Solder is used to hold the TE dice in place between two alumina substrates, and in totality, serves to hold the TE module together. It's widely known that when unprotected, solder migration into the TE dice can cause a degradation in the materials ability to serve as a semiconductor, resulting in deteriorating performance.
In order to mitigate solder migration into the TE dice, Crystal Ltd. uses a RoHS compliant Nickel anti-diffusion barrier that's protected from oxidation on the outer surface with a layer of Tin (Sn + less than 1% Bi). The thickness of the Ni deposition and protective layers vary, depending on the type of module they'll be used in, but are typically in the 4 to 10 micron range. After years of investigation, the company has improved both the anti-diffusion coatings and the and methods of their application. For modules with higher temperature ratings, the bond of the Nickel anti-diffusion barrier to the TE material is enhanced with an adhesion layer of Molybdenum. The result is that the reliability of Crystal Ltd.'s thermoelectric modules is outstanding, including those used for high temperature and thermal cycling applications.